Hot melt adhesives are widely used in industry for various applications such as product assembly and packaging, including particularly, for use in cardboard case sealing and carton closing operations. Some of these operations (e.g., for cartons, cases or trays used for packaging molten cheese at 60.degree. C. and its subsequent storage at freezer temperatures of -18.degree. to -6.degree. C. or for packaging yogurt or freshly baked goods at temperatures greater than about 40.degree. C.) require a hot melt adhesive with exceptionally high heat resistance (ability to maintain a fiber tearing bond at elevated temperatures) without sacrificing good cold resistance (ability to maintain a high strength bond in the cold with no tendency to fracture). Moreover, the viscosity of such adhesives must be low enough and the set speed fast enough to give good machinability, for example, on rapid-fire, automatic equipment with short compression sections which are used in commercial case or carton operations. The thermal stability and aesthetics also must be such as to produce no charring, skin, or gel formation coupled with a small viscosity change following prolonged aging at typical operating temperatures (e.g., 200.degree. C.) so as to minimize down time for maintenance and to provide consistent application patterns and amounts during operation. These aesthetic considerations have become more important in recent years as customers increasingly demand hot melt adhesives having pot clarity, i.e., adhesives which are clear in their molten form.
Hot melt case and carton sealing products available and in commercial use today (primarily polyethylene and ethylene vinyl acetate formulations) lack one or more of the previously described properties; (1) high temperature end use performance; (2) low temperature end use performance; (3) clean machining; (4) low viscosity; (5) desirable aesthetics (clarity); (6) satisfactory thermal stability; (7) and/or low cloud point (below 120.degree. C.). It would therefore be an advance in the art to provide another hot melt adhesive having good bond strengths (i.e., producing fiber tear) at both high and low temperatures coupled with low viscosity, fast set speed and excellent thermal stability, machining and aesthetics.
U.S. Pat. Nos. 4,816,306 and 4,874,804 to Brady, et al., disclose that hot melt packaging adhesive compositions consisting essentially of 35 to 45% by weight of an ethylene n-butyl acrylate copolymer containing 25-45% by weight n-butyl acrylate and having a melt index of at least 50; 35 to 55% of a terpene phenolic tackifier; and 10 to 20% by weight of a high melting point synthetic wax provide adhesives characterized by an excellent balance of high and low temperature performance without sacrifice to its machinability or thermal stability. While the thermal stability properties of these adhesives are similar to commercial EVA based hot melt adhesives and are acceptable for a number of end use applications, there exist end use applications which require repeated heating and cooling of the hot melt adhesive during application where these products show poor stability. In those cases, the repeated heating and cooling has been found to produce an unacceptable level of charring in the molten adhesive pot.
Moreover, in the case of applications wherein the final product, e.g., the case or carton, will be subjected to recycling operations after use, the adhesives of the U.S. Pat. Nos. 4,816,306 and 4,874,804 have a density approximately the same as that of the recycled pulp and, as a result, filtration methods cannot be used to separate the adhesive from the pulp. The presence of any residual adhesive, often referred to as "stickies", substantially reduces the value of the recycled pulp.